1. Field of the Invention
The present invention relates to visual displays, particularly to addressable, reusable, paper-like visual displays, and more particularly to gyricon or twisting-ball displays.
2. Discussion of Related Art
A gyricon display, also called the twisting-ball display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic balls, each of which can be selectively rotated to present a desired face to an observer. For example, a gyricon display can incorporate balls each having two distinct hemispheres, one black and the other white, with each hemisphere having a distinct electrical characteristic (e.g., zeta potential with respect to a dielectric fluid) so that the balls are electrically as well as optically anisotropic. The black-and-white balls are embedded in a sheet of optically transparent material, such as an elastomer layer, that contains a multiplicity of spheroidal cavities and is permeated by a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities accommodate the balls, one ball per cavity, so as to prevent the balls from migrating within the sheet. A ball can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either the black or the white hemisphere to an observer viewing the surface of the sheet. Thus, by application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the balls can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image.
An exemplary gyricon display 10 is shown in side view in FIG. 1. Bichromal balls 11 are disposed in an elastomer binder of the sheet 12 that is swelled by a dielectric fluid creating cavities 13 in which the balls 11 are free to rotate. The balls 11 are electrically dipolar in contact with the fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes 14a, 14b. The electrode 14a closest to viewing surface 15 is preferably transparent. An observer at I sees an image formed by the black and white pattern of the balls 11 as rotated to expose their black or white faces (hemispheres) to the viewing surface 15 of sheet 12.
U.S. Pat. No. 5,389,945, incorporated by reference herein, shows that gyricon displays can be made that have many of the desirable qualities of paper, such as flexibility and stable retention of a displayed image in the absence of power, not found in CRTs, LCDs, or other conventional display media. Gyricon displays can also be made that are not paper-like, for example, in the form of rigid display screens for flat-panel displays.
Typically, known gyricon displays are made up of bichromal balls that are black on one hemisphere and white on the other. Other kinds of rotating elements are also known. For example, U.S. Pat. No. 4,261,653 shows a multilayer sphere, although it is made at least in part from glass and its use depends on an addressing scheme involving high-frequency electric fields.
Other commonly owned patents related to gyricon displays, each incorporated herein by reference in their entireties, include U.S. Pat. Nos. 5,262,098; 5,344,594; 5,717,514; 5,815,306; 5,989,629 and 6,097,531.
In making the gyricon sheet of the gyricon display, the binder, typically an elastomer, of the sheet is soaked in the dielectric fluid after it is cured. This swells the binder to create the cavities 13 slightly larger than and surrounding the spheres 11, which cavities are filled with the dielectric fluid.
What is still needed is a reliable method of controlling the extent of the swelling of the gyricon sheet.
It is therefore an object of the present invention to develop a method of making a gyricon sheet in which the swelling of the sheet with a dielectric fluid is precisely controlled.
It is a further object of the present invention to achieve a gyricon sheet that exhibits a high display brightness.
These and other objects of the present invention are achieved herein by swelling the binder of the gyricon sheet with dielectric fluids comprised of a mixture of at least two liquids, wherein the liquids possess different levels of swelling the binder, i.e., possess different binder swelling capabilities. In particular, a first liquid is a liquid that swells the binder to a higher degree than a second liquid. The second liquid may not swell the binder at all, or may swell the binder to only a relatively small degree.
The invention also pertains to a method of making a gyricon sheet in which the mixture of at least two liquids comprises at least one volatile liquid having a binder swelling capability and at least one non-volatile liquid having no binder swelling capability, and wherein after swelling the binder with the mixture, the method further comprises evaporating off the at least one volatile liquid. In this method, the resulting gyricon sheet contains only the non-volatile non-swelling dielectric liquid.
The invention also pertains to a gyricon sheet comprising a binder containing rotating elements, wherein the binder is swollen with dielectric fluids so as to have cavities larger than the rotating elements around each of the rotating elements, the cavities filled with the dielectric fluids, and wherein the dielectric fluids comprise a mixture of at least two liquids having different binder swelling capabilities.
By selection of the mixture of dielectric liquids, the extent of swelling of the gyricon sheet in the process can be precisely controlled, enabling the gyricon sheet to exhibit a high level of display brightness.